It has heretofore been proposed, for example as shown in U.S. Pat. Nos. 2,302,449; 2,302,450 and 3,239,061, to make a filter apparatus in which the filter bed is supported on a generally horizontal porous support in a tank and partitions provided at spaced locations along the tank to separate the bed into a plurality of filter cells above the porous support and to divide the space below the porous support into a plurality of filtrate compartments the communicate at one end through a filtrate port with a filtrate launder that extends along the length of the tank. In such filter apparatus, the fluid to be filtered is supplied to the tank above the bed of filter material and passes downwardly through the bed of filter material into the filtrate compartments and out of the filtrate compartments into the filtrate launder to a discharge outlet. The bed of filter material was progressively cleaned by a carriage mounted for movement along the bed and having a backwash apparatus including a backwash head movable through the filtrate launder sequentially into communication with the filtrate ports to supply backwash fluid to the filtrate compartments at the underside of the porous support and a collector hood carried by the carriage and movable along a path over the top of the beds to collect the backwash effluent. The silt and solids that collect on the surface of the filter bed tends to form a relatively dense layer over the top of each filter cell and, when pressured backwash water is introduced under a filter cell, this dense layer on the filter cell can rupture at a location externally of the hood and blow the deposited material back into the fluid to be filtered. In order to aid in dislodging the slit and solids that collect on the surface of the filter bed, it has also been proposed, for example as shown in the aforementioned U.S. Pat. No. 3,239,061, to provide a mechanical agitator or scarifier in the collector hood for loosening the surface of the filter bed. However, the scarifier shown in that patent is located at the centerline of the hood so that it would not function to agitate the material on the surface of the filter cell until some time after the backwash head moved into communication with the cell and supplied pressurized backwash fluid thereto.
In prior filter apparatus of the type shown in the aforementioned U.S. Pat. Nos. 2,302,449; 2,302,450 and 3,239,061 which had a traveling backwash head for introducing backwash fluid under pressure into the filtrate compartments below the porous filter bed support, the backwash head was arranged to slidably engage a guide surface extending along the locus of the filtrate ports to seal the adjacent filtrate ports from the filtrate launder and inhibit leakage of fluid delivered from the backwash pump. The sliding shoe imposed a drag on the carriage and, further, the shoe and the guide surface engaged thereby are subject to wear.